1. Field of the Invention
This invention relates to molding a material of one density, around or sandwiched with lower density materials, to yield a single composite with significantly improved physical properties than those of the individual components.
2. Discussion of Prior Art
The conventional injection molding process is a high pressure process which has never been conducive to molding thick wall sections with or without integral delicate features or light weight cores. This is mostly due to the fact that conventional injection molding is a high pressure process employing injection pressures up to several thousand pounds per square inch (psi).
While the more recent invention of gas-assisted injection molding has proven useful for achieving lower cavity pressure during the filling phase, the process only benefits thin-walled parts which have thicker gas runners built in to the geometry. Gas runners are used with the gas-assist process specifically to allow a path for the gas to flow into. Gas injected into a part being molded will core out the thicker internal regions which are molten, while the surfaces touching the walls of the mold freeze off, forming parts with hollow channels in the gas runners. The resulting parts exhibit excellent physical properties, not to mention lower cost due to less material and (consequently) smaller molding machinery.
The largest problem facing the gas-assisted injection process to date is that it is virtually impossible to predict how the gas bubble will form inside the gas runner of a gas injection molded part. Equally important is the fact that parts with thick wall sections cannot benefit from gas-assist because the gas bubble formation would occur in a totally random formation, likely causing a "blow through", in which the gas pops through the side of the part. This "blow through" phenomena is analogous to a balloon bursting after being over inflated (you know it is going to break when it is over inflated, but you don't know exactly where the hole will be). Similarly, blow through may also occur even in properly designed gas-assist parts if the process conditions are not optimal. Equally as disturbing as gas blow through is the occurrence of gas fingering. Gas fingering occurs randomly during the gas-assist process. Instead of gas actually blowing out through the sidewall, it displaces material within the wall thickness of the part. This fingering effect can be very damaging to a part because it could occur even in parts that have been designed with gas runners, and you may not be able to actually see it if it occurs internally.
Studies in gas-assisted injection molding (GIM) thus far have not developed techniques for molding parts with multi-dimensional flow paths of varying lengths, geometries with thick walls combined with large surface areas, parts with separate inner cores, and parts that consist of materials other than (or in addition to) plastic. Although not a gas-assist process, the "lost core" technology has been the only technology thus far to offer parts of various shapes with good dimensional characteristics.
The process still does not provide a method for significantly increasing thermal, mechanical and acoustical properties of molded parts, nor does it provide a single molding process capable of producing complex geometric shapes with low weight and high strength. Moreover, the lost core process requires labor intensive secondary operations necessary to remove the inner core after parts have been molded.
The original motivation behind my process was to develop a synthetic polymer bone compound to be used in vehicle crash test simulations. The bone was to have similar properties and features as the human skull. Ideally, the bone was to consist of thick hollow sections with an integral skin and foam core. The project turned out to be very complicated and ultimately it was decided that a simplified geometry could be substituted for the skull which had the necessary qualities needed for crash test simulations. Later it was determined that the synthetic bone project had many features that would be difficult, if not impossible to accomplish by means of the conventional injection molding process.
The simplified geometry that I used contains a flat open cell (porous) foam core plate that is held in place by delicate pins in the mold. Material flowing into the cavity would have to flow around the inner core while filling very thick sections over a large surface area. The conventional injection molding process is a high pressure process which generates filling pressures up to several thousand pounds per square inch (psi). It was determined that these high cavity pressures would be detrimental to this part, and therefore gas-assisted injection molding became a likely alternative for molding the part.
A project by Matsushita Corporation found that the gas-assisted injection molding (GIM) process could be used to mold parts with delicate pins successfully. Therefore, the GIM process, being a low pressure process, became a feasible alternative for the bone project. It was believed that because this process produces very low filling pressures (frequently around 500 psi), this part, and similar thick-walled parts with large surface areas could be produced with good results. The only drawback was that this part contained geometric features that the current GIM studies and processes had not yet investigated.